Decades before "the dress" broke the internet, an optical illusion called “My Wife and My Mother-in-Law" had people second-guessing their eyesight. It depicts two images hidden in one illustration: The first shows a young woman turning her face away from the viewer, and the second shows the profile of a much older woman looking to the left. While both perceptions are technically accurate, the way you see the picture may give away your age, according to a study reported by indy100.

The study, published recently in Scientific Reports, looked at 393 subjects of varying ages. Researchers found that the youngest 10 percent of participants were more likely to see the young woman first when shown the illusion, while the oldest 10 percent of participants saw the old woman first.

Research has shown that people spend more time processing the faces of people they consider to be members of their "group" than those outside of it. "The effect of in- and out-groups may be particularly relevant to the processing of faces of different ages," the authors wrote of the study. "A common finding within the field is the own-age bias, where recognition memory is better for faces closer to one’s own age compared to other ages."

“My Wife and My Mother-in-Law" is an example of an ambiguous image or reversible figure—optical illusions that your brain can interpret one of two ways. These pictures are satisfying once you figure them out, but other optical illusions aren't so easy to outsmart.

[h/t indy100]